In transmitters where the radio frequency output power is attained by combining the outputs of as many as thirty to forty modules, it is desirable to monitor certain operating characteristics of each module. This can be done on a routine basis or in the event of a suspected malfunction of the transmitter. Whereas this can be accomplished by removing each module and checking it in a bench set-up, this requires an inordinate amount of time. Furthermore, there are circumstances where the malfunction can only be detected with the module in its operating position.